You can do this cheat after you’ve obtained 
W-Item Materia. Equip it to one of your characters and get into a battle. Make sure you’re battling a weak enemy to make this easier. Select the W-Item command on the character you equipped the Materia to.
Select an item that you can waste on a character (potions, ethers, anything will do). The menu will open allowing you to select the second item. Press Select on the item (don’t use it) and then Cancel the item selection. This will add that item to your inventory. Repeat this process until you have 99 of the item. This cheat only works on items you can select from the battle screen.
The best and most efficient way to earn money in the game is to level and sell Mastered All Materias as you obtain them. It doesn’t require any sidetracking from the missions at hand and every time an All is mastered you get a new one. The new one can’t be used as many times, but at late stages in the game it’s easy enough to get them leveled to level 2 or 3, which is almost as useful as the actual mastered 
All Materia. A Mastered All Materia sells for 1.4 million gil.
